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2 marks q&a

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2 marks q&a

1. 1. 2MARKS QUESTION & ANSWERS UNIT-1 DIGITAL COMMUNICATION SYSTEM 1. Define modulation? Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal. 2. What is correlative coding? Correlative level coding is used to transmit a baseband signal with the signaling rate of 2Bo over the channel of bandwidth Bo. This is made physically possible by allowing ISI in the transmitted in controlled manner. This ISI is known to receiver. The correlative coding is implemented by duobinary signalling and modified duobinary signalling. 3. State sampling theorem. · If a finite –energy signal g(t) contains no frequencies higher than W hertz ,it is completely determined by specifying its co=ordinates at a sequence of points spaced 1/2W seconds apart. · If a finite energy signal g(t) contains no frequencies higher than W hertz, it may be completely recovered from its co=ordinates at a sequence of points. 4. What are the degrees of modulation? Under modulation. m<1 Critical modulation m=1 Over modulation m>1 5. What is aliasing? The phenomenon of a high-frequency in the spectrum of the original signal g(t) seemingly taking on the identity of a lower frequency in the spectrum of the sampled signal g(t) is called aliasing or foldover. 6. Why do you need adaptive equalization in a switched telephone network. In switched telephone network the distortion depends upon 1) Transmission characteristics of individual links. 2) Number of links in connection. Hence fixed pair of transmit and receive filters will not serve the equalization problem. The transmission characteristics keep on changing. Therefore adaptive equalization is used. 7.What are the necessity of adaptive equalization?
2. 2. Ans. Most of the channels are made up of individual links in switched telephone network, the distortion induced depends upon 1) transmission characteristics of individual links 2) number of links in connection 8. Define the principle of adaptive equalization? Ans. The filters adapt themselves to the dispersive effects of the channel that is the cofficients of the filters are changed contineously according to the received data. The filter cofficients are changed in such a way that the distortion in the data is reduced 9. Define duobinary encoding? Ans. Duobinary encoding reduces the maximum frequency of the base band signal the “word duo” means to the double transmission capacity of the binary system 10. Write a note on correlative level coding? Correlative level coding allows the signal scaling rate of 2Bo in the channel of bandwidth Bo this is made physically possible by allowing ISI in the transmitted signal in controlled manner this ISI IS KNOWN TO THE RECEIVER 11. Define the term ISI? Ans. The presence of outputs due to other bits interference with the output of required bit . this effect is called inter symbol interference (ISI) 12. Write the performance of data transmission system using eye pattern technique? Ans. The width of the eye opening defines the interval over which the received wave can can be sampled without error from inter symbol interference . The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied. 13. What is the necessity of equalization? Ans. When the signal is passed through the channel distortion is introduced in terms of 1) amplitude 2) delay this distortion creates problem of ISI. The detection of the signal also become difficult this distraction can be compensated with the help of equalizer. 14. What is meant by prediction error? The difference between the actual sample of the process at the time of interest and the predictor output is called a prediction error. 15. Define delta modulation. Delta modulation is the one-bit version of differential pulse code modulation.
3. 3. 16. Define adaptive delta modulation. The performance of a delta modulator can be improved significantly by making the step size of the modulator assume a time- varying form. In particular, during a steep segment of the input signal the step size is increased. Conversely, when the input signal is varying slowly, the step is reduced , In this way, the step size is adapting to the level of the signal. The resulting method is called adaptive delta modulation (ADM). UNIT -2 : BASEBAND FORMATTING TECHNIQUES 1. Mention the need of optimum transmitting and receiving filter in baseband data transmission. When binary data is transmitted over the baseband channel, noise interfaces with it. Because of this noise interference, errors are introduced in signal detection. Optimum filter performs two functions while receiving the noisy signal: 1) Optimum filter integrates the signal during the bit interval and checks the output at the time instant where signal to noise ratio is maximum 2) Transfer function of the optimum filter is selected so as to maximise signal to noise ratio. 3) Optimum filter minimizes the probability of error. 2. Define ASK. In ASK, carrier is switched on when binary 1 is to be transmitted and it is switched off when binary D is to be transmitted ASK is also called on-off keying. 3. What is meant by DPSK? In DPSK, the input sequence is modified. Let input sequence be d(t) and output sequence be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0. When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence the technique is called Differential PSK. 4. Explain coherent detection? In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. The detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a synchronous detection.